1. Field of the Invention
The invention is directed to an improved process for producing ethylene-higher alpha olefin elastomers. More particularly, the invention provides for the manufacture of EPDM elastomers having relatively broad molecular weight distribution, while having a very narrow compositional distribution.
2. Description of the Prior Art
Many Ziegler-type catalyst systems are known, the most common of which are the vanadium and titanium halides, either used alone or in combination, with the various organoaluminum reducing agents.
By using various combinations of these Ziegler catalyst systems, one is able to control molecular weight distribution, gel content, monomer sequence distribution and amount within the given EPDM elastomer, as well as many other polymer properties. Within this art, there is a correlation between weight average molecular weight and number average molecular weight, known as "polydispersity" (Q). Expressed empirically, this relationship is M.sub.2 /M.sub.n. Q is known to range as high as about 20, which indicates a very broad molecular weight distribution within a given polymer sample.
This property, along with molecular weight, and polymer composition, directly effect the processability of the elastomer. However, until the present invention, it was not known how to obtain an EPDM elastomer containing a certain combination of properties. This combination is a broadened molecular weight distribution and a narrow compositional distribution.
It has been shown that certain Ziegler catalysts could be reactivated by using a series of at least three reactors. The EPDM reaction is conducted in all three reactors by adding all the catalyst to the first reactor and adding a reactivator to the subsequent stages. A recently issued patent with this teaching is U.S. Pat. No. 3,629,212. This method of production has the disadvantage of added cost of reactivator and purification problems.